The present invention relates generally to an electric power converter comprising both controlled and uncontrolled electric valves and having forced commutation means for turning off the controlled valves, and it relates more particularly to an improved feature of the controls for a converter well suited to supply direct current (d-c) of variable magnitude to the d-c traction motors on an electric locomotive that is energized from a wayside source of alternating voltage.
Large electrically driven land vehicles such as locomotives and transit cars are propelled by a plurality of traction motors whose rotors are mechanically coupled through speed-reduction gearing to the respective axle-wheel sets of the vehicle. Such motors are usually of the d-c type. If the vehicle is intended to travel along an electrified right of way, it is equipped with a current collector (e.g., a pneumatic or spring-loaded pantograph on the roof of the vehicle) that makes sliding or rolling contact with a bare conductor (e.g., an overhead catenary) extending parallel to the rails or guideway defining the traction vehicle's path of movement. The overhead conductor is part of a power distribution system comprising a source of high-voltage electric power that supplies alternating current (a-c) at a standard frequency such as 60 Hz in the United States or 50 Hz in Europe.
In order to convert the a-c power that is available from the wayside distribution system into direct current of variable magnitude suitable for energizing the armature and field windings of the d-c traction motors on the vehicle, the propulsion system includes a voltage step-down power transformer and a controllable electric power converter. The transformer comprises a single-phase, high-voltage primary winding connected between the current collector and the vehicle wheels (which are at ground potential) and a plurality of lower voltage secondary windings. The converter comprises two or more single-phase, full-wave rectifying bridges. Each bridge has two load-current carrying branches or legs connected in parallel with one another between a pair of output terminals which are connected to at least one of the traction motors, and each leg of the bridge comprises at least one pair of serially connected unilaterally conducting electric valves. A transformer secondary winding is connected across the junctures of the respective valve pairs in the two legs of each bridge, thereby applying alternating voltage to the bridge. The bridge is effective to convert the applied alternating voltage to direct voltage at its output terminals.
The valves in at least one leg of each rectifying bridge in the propulsion system of an electric locomotive are usually uncontrolled devices or simple diodes. In order to vary the output voltage of at least one of the bridges, the other leg of that bridge comprises a pair of serially connected, periodically conducting controllable valves (thereby forming a semi-controlled asymmetrical or "hybrid" rectifying circuit). The average magnitude of the output voltage can then be varied as desired by suitably varying, in synchronism with the applied voltage, the "conduction angles" of such controllable valves (i.e., the lengths of their conducting periods, measured in electrical degrees). During intervals when neither of the controllable valves is in a conducting state, there is no current in the associated secondary winding and motor current will coast or "free-wheel" through a diode leg of the rectifying bridge.
For maximum efficiency the controls of the propulsion system on a vehicle such as an electric locomotive are conventionally designed to work the traction motors at substantially constant horsepower throughout a wide speed range of the locomotive. This is usually done by regulating the magnitude of motor current (and hence motor torque) so that it varies inversely with approximately the square root of locomotive speed as the latter varies between a predetermined "corner point" speed and rated maximum speed. But from zero speed to the corner point speed, maximum current is desired in order to provide high tractive force or effort for accelerating the locomotive from rest. In this low speed range, the voltage applied to the armature windings of the traction motors is relatively low because the counter emf of each motor, which is proportional to speed, is relatively low. At maximum speed maximum voltage must be applied to the motors to overcome their high counter emf, while the magnitude of motor current can now be relatively low because the motors draw less current at high speed than at low speed.
To meet the above-reviewed requirements of both low speed and maximum speed propulsion, it has heretofore been common practice to "stage" the rectifying bridges so that initially, as the locomotive is accelerating from rest, the direct voltage applied to a traction motor is provided by a single bridge (which includes controllable valves) and so that later, when the locomotive attains full speed, the motor voltage is the sum of voltages individually contributed by two or more rectifying bridges whose output terminals are interconnected in series. One well known way to do this is to use at least first and second hybrid bridges in series and to control them in sequence so that during the first stage the conduction angle of each controllable valve in the first bridge is continuously increased from zero to maximum (approximately 180 electrical degrees) while motor current passes through the diode leg(s) of the other bridge(s), thereby increasing the average magnitude of motor voltage from zero to a level equalling the maximum output voltage of the first bridge, whereas during a second stage the conduction angle of each controllable valve in the second bridge is similarly increased while maintaining a maximum conduction angle in the first bridge, thereby further increasing motor voltage to a higher level which equals the maximum output voltage of the first bridge plus the output voltage of the second bridge.
In the present state of the art, the main load-current carrying electric valves in the controllable leg of the hybrid rectifying bridge are high-power, solid-state controllable switching devices known as thyristors or semiconductor controlled rectifiers (SCRs), and the valves in the uncontrolled leg are power rated PN semiconductor diodes having a single rectifying junction. A thyristor is typically a three-electrode PNPN device having an anode, a cathode, and a control or gate terminal. When its anode and cathode are externally connected in series with an electric power load and a source of forward anode voltage (i.e., anode potential is positive with respect to cathode), a thyristor will ordinarily block appreciable load current until a firing or trigger signal is applied to the control terminal, whereupon it switches from its blocking or "off" state to a conducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. The time at which the thyristor is turned on, measured in electrical degrees from a cyclically recurring instant at which its anode voltage becomes positive with respect to its cathode at the start of the appropriate half cycle of alternating voltage applied to the bridge, is known as the "firing angle." The average magnitude of the output voltage of a hybrid bridge can be varied by retarding or advancing the firing angle as desired. This is popularly known as "phase control." Hereinafter the firing angle is sometimes also referred to as the "ignition angle."
Once a thyristor is turned on, it can be turned off only by reducing its current below a given holding level and applying a reverse voltage across the anode and cathode for a time period sufficient to allow the thyristor to regain its forward blocking ability. In a conventional line-voltage commutated phase-controlled hybrid bridge, a conducting thyristor is naturally turned off at the end of each half cycle of the applied voltage, at which time the corresponding diode in the parallel diode leg of the bridge becomes forward biased and current transfers from the thyristor to the diode. Such current transfer is referred to as commutation, and the length of the commutation interval (when both the outgoing or relieved thyristor and the incoming or relieving diode are simultaneously conducting) will depend on the magnitude of current being commutated and the inductance (including the leakage reactance of the transformer secondary winding) in the paths of changing current.
Phase-controlled rectifier operation imposes a lagging power factor load on the a-c source. In other words, when the ignition angle of the thyristor leg in a conventional hybrid bridge is delayed or retarded from a fully advanced condition, the fundamental component of alternating current in the transformer primary winding tends to lag the fundamental component of the source voltage, and the locomotive draws undesired reactive or apparent power in addition to useful real power from the wayside distribution system. This is particularly objectionable when the propulsion system is in a high constant horsepower mode and the locomotive is traveling through a region of the electrified right of way that is relatively remote from the nearest wayside substation. To reduce the reactive component of power and consequently to improve the power factor of an electric locomotive (i.e., to obtain a power factor that approaches unity), it has heretofore been proposed to provide means for periodically turning off the main thyristor leg of one of the hybrid rectifying bridges prior to the time at which commutation would otherwise naturally occur. For this purpose, turn off means known as a forced commutation circuit can be used. Such means enables the conducting thyristor to be quenched or turned off at any desired "extinction angle." The extinction angle, which is measured in electrical degrees from the cyclically recurring negative-to-positive zero crossing of the anode voltage on the main thyristor, marks the time at which suitable action is initiated to turn off the main thyristor and thereby extinguish its current. By appropriately controlling both the ignition angle and the extinction angle of the thyristors in one bridge, the average magnitude of the net output voltage that is applied to the traction motors can be varied as desired while the fundamental component of transformer primary current is positioned to remain nearly in phase with the fundamental component of primary voltage.
A typical forced commutation circuit comprises at least one pair of auxiliary thyristors connected in circuit with at least one commutating capacitor across the respective main thyristors of the rectifying bridge so that each main thyristor in turn is forced to turn off by triggering a complementary one of the auxiliary thyristors. In operation, upon turning on the appropriate auxiliary thyristor the voltage on the precharged commutating capacitor is connected across the then conducting main thyristor with reverse polarity, and load current is quickly diverted from the main thyristor to a parallel path provided by the commutating capacitor and auxiliary thyristor. Thus current is transferred or commutated from the main thyristor to the auxiliary thyristor. After its current is reduced to zero, the main thyristor is temporarily reverse biased by the voltage across the discharging commutating capacitor, and it regains its forward blocking ability during this reverse bias or turn off interval. The extinction angle of the main thyristor therefore coincides with the firing of the auxiliary thyristor.
The aforesaid turn off interval ends when the commutating capacitor is fully discharged. Thereafter load current recharges the commutating capacitor with opposite polarity, and forward anode voltage is reapplied to the turned off main thyristor. The capacitor voltage will soon rise to a magnitude exceeding that of the applied alternating voltage, whereupon the difference therebetween forward biases a previously non-conducting diode in the same rectifying bridge. As a result, load current is now commutated or transferred from the auxiliary thyristor to the diode leg of the bridge. Consequently the auxiliary thyristor turns off and the commutating capacitor is left with a charge of proper polarity for subsequently forcing the other main thyristor to turn off when its complementary auxiliary thyristor is triggered during the succeeding half cycle of operation.
The principle of using forced commutation to obtain unity power factor is further explained in prior art U.S. Pat. No. 3,392,319. Various forced commutation circuits are known, as illustrated, for example, by U.S. Pat. Nos. 3,849,718 and 4,181,932. The ability to control the turn off of the main valves in one of the rectifying bridges of an electric locomotive enables the locomotive to be operated at a desirably high power factor. A high power factor results in lower power losses and less voltage droop, thereby reducing the required size and cost of not only the power transformer on board the locomotive but also the transformers and generators in the wayside electric power system. However, designing a locomotive propulsion system using forced commutation to improve its power factor has necessitated finding solutions to several practical problems.
One problem with a hybrid bridge that is provided with forced commutation is the possibility that, if the desired extinction angle were only slightly larger than the ignition angle, the reverse recovery voltage that is generated across a diode could rise to a destructively high magnitude. Reverse recovery voltage is associated with the turn off process of semiconductor valves. At the end of each commutation interval during which load current transfers to the most recently turned on main thyristor from the corresponding diode, the diode is unable to withstand reverse voltage and to block reverse current until a recombination phenomenon takes place in its PN junction. This entails a brief period of reverse current in the diode. As the diode regains its blocking capability, the reverse current rapidly decays to zero and a high reverse recovery voltage is generated. If this voltage exceeds a safe maximum magnitude or peak, it can destroy the diode.
To limit peak reverse recovery voltage, it is common practice to provide a series resistor-capacitor circuit known as a snubber in shunt with each diode. In addition, the amplitude of the reverse recovery current can be desirably reduced by providing a saturable reactor in series with the diode and/or by using a special "fast recovery" diode, but implementing these additional measures to prevent reverse recovery overvoltage increases the cost of the converter. The cost increment could be prohibitively high in a high current converter (such as is used in a high power electric locomotive) where a plurality of diodes are connected in parallel with each other to obtain the requisite high current rating.